Functional and molecular characteristics of system l in human breast cancer cells

Abstract

The functional and molecular properties of system l in human mammary cancer cells (MDA-MB-231 and MCF-7) have been examined. All transport experiments were conducted under Na +-free conditions. α-Aminoisobutyric acid (AIB) uptake by MDA-MB-231 and MCF-7 cells was almost abolished by BCH (2-amino-2-norbornane-carboxylic acid). AIB uptake by MDA-MB-231 cells was also inhibited by l-alanine (83.6%), l-lysine (75.6%) but not by l-proline. Similarly, l-lysine and l-alanine, respectively, reduced AIB influx into MCF-7 cells by 45.3% and 63.7%. The K m of AIB uptake into MDA-MB-231 and MCF-7 cells was, respectively, 1.6 and 8.8 mM, whereas the V max was, respectively, 9.7 and 110.0 nmol/mg protein/10 min. AIB efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH, l-glutamine, l-alanine, l-leucine, l-lysine and AIB (all at 2 mM). In contrast, l-glutamate, l-proline, l-arginine and MeAIB had no effect. The interaction between l-lysine and AIB efflux was one of low affinity. The fractional release of AIB from MDA-MB-231 cells was trans-accelerated by d-leucine and d-tryptophan but not by d-alanine. MDA-MB-231 and MCF-7 cells expressed LAT1 and CD98 mRNA. MCF-7 cells also expressed LAT2 mRNA. The results suggest that AIB transport in mammary cancer cells under Na +-free conditions is predominantly via system l which acts as an exchange mechanism. The differences in the kinetics of AIB transport between MDA-MB-231 and MCF-7 cells may be due to the differential expression of LAT2.